Suitable GPU (Dual GPUs!) for Precision T5500

Hey guys  :)
I have a Dell Precision T5500 (as the title says) with these specs:


Intel 5520 Chipset – DELL 0CRH6C Board (Bios: A18)
Standard 875W Power Supply (Model: H875EF-00)
2x Intel Xeon X5660 @2.80Ghz 
72 GB RAM – DDR32x PNY Quadro P620 2GB

I was wondering what kind of GPU I should buy, to upgrade my dual NVIDIA Quadro P620?
Here's the thing:
I have 6 monitors.   Most GPUs come with 3 or 4 outputs, so I need 2 GPUs.
And here's the catch:
The SPECSHEET for the T5500 mentions this, regarding the supported graphic cards:
"Support for 2 PCI Express x16 Gen 2 graphics cards up to 150W and with up to 4GB graphics memory"


150W?  Is this really important, if I have a 875W Power Supply?
What about the 4GB?  What happens if I buy 2 GPUs with 8GB or 16GB each?

Here are some GPUs I was considering:

- 2x AMD RADEON PRO WX7100 8GB (130W each)
- 2x NVIDIA Quadro P4000 8GB (105W each)
- 2x NVIDIA Quadro P5000 16GB (180W each)
- 2x NVIDIA Quadro RTX 5000 16GB (230W each)

Another option would be to get:
- 1x Dell AMD Radeon Pro WX9100 16GB (230W)
(since it already has 6 outputs)

I'd like to buy the most powerful GPUs, the reason is, I intend to use Stable Diffusion and it requires a lot of GPU power.
If Stable Diffusion is not an option (is my workstation too old?) I will then consider any suggestion to upgrade my actual GPUs (the P620s definitely suck!)

Thanks guys!
Lazy Geek  :D